Image display devices such as projection-type displays and so on generally use an optical scanning device that scans light. Conventionally, optical scanning devices of this kind have used a motor-driven polygon mirror, galvano mirror and so on.
Meanwhile, accompanying the advancement of micromachining technologies in recent years, optical scanning devices that make applied use of MEMS technology have developed significantly. Of this, optical scanning devices that scan light by making an optical scanning mirror oscillate back and forth by using a beam part as a rotating axis, have been gaining attention. This kind of optical scanning mirror is structured simple compared to a conventional motor-driven polygon mirror and so on and can be formed as one entity by semiconductor processing, so that there are advantages that this optical scanning mirror can be miniaturized, at lower cost, and furthermore this miniaturization makes possible higher speed, and so on.
An optical scanning mirror according to MEMS technology is generally driven by matching the resonance frequency of the structure and driving frequency, in order to increase the deflection angle (resonance drive).
Given that the torsional modulus of elasticity of the beam part is k and the inertia moment of the optical scanning mirror is Im, the resonance frequency fr of the optical scanning mirror is given by following equation (1):fr=1/(2π)·(k/Im)1/2   (1)
Given that the driving force that is applied to the optical scanning mirror is T, the deflection angle θ of the optical scanning mirror in resonance drive is given by following equation 2:θ=QT/k   (2)
In equation (2), Q is the quality factor of the system, typically having a value of approximately 100 in the air and typically having a value of approximately 1000 in a vacuum.
Consequently, it is possible to make an optical scanning mirror in resonance drive swing big by comparatively small driving force.
On the other hand, according to an optical scanning device of one kind, the above-described optical scanning mirror is driven without matching the resonance frequency of the structure and driving frequency (non-resonance drive).
The deflection angle θ of the optical scanning mirror in non-resonance drive is given by following equation (3):θ=T/k   (3)
According to equation (3), it is not possible to employ the quality factor Q, and therefore, compared to equation 2, the deflection angle θ of the optical scanning mirror is small. So, to increase the deflection angle θ of the optical scanning mirror, it is necessary to increase the driving force T or decrease the torsional modulus of elasticity k of the beam part. However, as derived from equation 1, making the torsional modulus of elasticity k of the beam part smaller results in making the resonance frequency fr of the optical scanning mirror lower. Then, there are also cases where, when the resonance frequency fr of the optical scanning mirror comes close to the driving frequency in non-resonance drive (normally 60 Hz), resonance mode overlaps the oscillating waveform of the optical scanning mirror. To prevent this, typically, it is necessary to set the resonance frequency fr of the optical scanning mirror to around 1 kHz. Consequently, to increase the deflection angle θ of the optical scanning mirror, it is preferable to increase the driving force T, rather than decrease the torsional modulus of elasticity k of the beam part.
As such optical scanning devices of non-resonance type, optical scanning devices using a magnetic force drive device are known.
For example, patent literatures 1 and 2 disclose optical scanning devices using a moving coil-type (MC-type) magnetic force drive device. These optical scanning devices mount a coil in an optical scanning mirror that is placed between a plurality of permanent magnets, and drive the optical scanning mirror utilizing the Lorentz force that is produced by applying a current to this coil.
Also, patent literatures 3 and 4 disclose optical scanning devices using a moving magnet-type (MM-type) magnetic force drive device. These optical scanning devices mount a permanent magnet on an optical scanning mirror and drive the optical scanning mirror by utilizing the magnetic interaction that is produced by applying a current to a coil that is placed near the optical scanning mirror.